Anodized aluminum must be subsequently sealed to fill the pores in the anodic oxide formed by the anodizing treatment. In its simplest form, such sealing may comprise treatment with hot water.
However, commercial problems associated with maintaining a good quality seal with water, such as contamination, pH fluctuation and sealing smudge, have necessitated improvements in the sealing process. Such improvements have been directed towards efforts to produce a high quality seal while eliminating an undesirable chalk-like smudge which is created during the sealing process and is most noticeable with the dark colors produced by the newer anodizing processes.
The initial solution to this problem, described in Alexander U.S. Pat. No. 3,791,940, was to seal the anodized aluminum with a hydrolyzable metallic salt such as cobalt or nickel acetate which formed a smudge which could be removed with a mineral acid.
Subsequently, it was disclosed in Meyer et al U.S. Pat. No. 3,897,287 that this sealing process could be facilitated by the addition of from 30 to 2000 ppm of a soluble sulfate, such as SO.sub.4, which in its simplest form, comprised the addition of sulfuric acid to the sealing solution.
Further attempts at modification of the sealing solution have been made, with varying degrees of success. For example, Remaley et al U.S. Pat. No. 4,045,599 suggests the addition of triethanolamine to the sealing solution to enable sealing to occur at a lower bath temperature, while Treiber U.S. Pat. No. 3,689,379 suggests the use of a dispersant with the hydrolyzable salt to form a seal said to not leave a smudge.
This later patent omits the use of the soluble sulfate SO.sub.4 ion taught by the aforesaid Meyer et al patent. The Treiber patent teaches instead the use of 35 to 60 wt. % of a sodium salt of an alkyl naphthalene sulfonic acid dispersant with 35 to 60 wt. % of nickel or cobalt acetate with 0.03 to 0.3 wt. % of a sodium lauryl sulfate wetting agent and 0.1 to 1.0 wt. % of octyl phenoxy ethanol wetting agent.
While a method of sealing which does not require a subsequent desmudging is attractive, experience has shown that such is not, in fact, easily attainable. Furthermore, the advantages of omission of additional steps in a process usually are based on economic considerations. Substitutions of large amounts of expensive wetting or dispersing agents for an additional process step can, therefore, result in an economic standoff.